Motor vehicles equipped with gasoline engines have fuel vapor recovery systems that collect fuel vapors from the fuel tank or fuel tank filler and store the vapors in a canister containing activated charcoal. On occasion a vapor purge operation is performed where fresh air enters the canister, desorbs stored fuel vapors, and the vapors are then inducted into the engine for combustion.
A diagnostic test also is performed on occasion to determine if there are vapor leaks in the fuel vapor recovery system. The system is sealed from atmosphere and a negative pressure created by drawing in vapors through the engine air intake until a predetermined pressure is reached. Thereafter pressure measurements are taken to determine if there is a leak.
The fuel vapor recovery system also contains a negative pressure relief valve that releases pressure at a preset negative pressure to prevent too large a negative pressure forming in the fuel vapor recovery system or fuel tank.
The inventors herein have recognized a potential problem with such systems, particularly in hybrid electric or plug-in hybrid applications. Under some driving conditions the internal combustion engine, and vapor purging, may not operate for a long period of time. Hydrocarbons may therefore be more likely to form in the pressure relief valve causing it to stick and not release at the preset negative pressure. The inventors herein have solved this problem, in one particular approach, by reducing fuel system pressure to a predetermined pressure which is higher than the preset pressure during a test or diagnostic cycle, and when not in the test cycle, periodically reducing the fuel system pressure to a third pressure which is less than the preset release pressure to unstick the pressure relief valve.
In another approach, a method for performing operations on a fuel vapor system having a vapor absorbing canister coupled between a fuel tank and an engine intake via a purge valve, and a pressure relief valve which opens at a preset negative pressure to limit maximum negative pressure in the fuel vapor system is described comprising a test operation, a vapor purge operation, and a pressure relief valve cleaning operation.
The test operation reduces fuel system pressure to a predetermined pressure which is higher than the preset pressure, measures pressure changes in the fuel system, and indicates a fault when the measurements exceed a threshold. The vapor purge operation purges fuel vapors from the fuel vapor system into the engine intake manifold by operating the purge valve, and a pressure relief valve cleaning operation reduces the fuel system pressure to a third pressure which is less than the preset pressure to force the relief valve open even when stuck closed.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.